1. Field of the Invention
The invention relates to a process for the preparation of hydrophobic silicas.
2. Description of the Related Art
The surface modification of finely divided silicas with dimethyldichlorosilane in a fluidized bed is known (DE 1163784). Here, hydrophobic silicas which, in addition to a considerable proportion of unreacted silica silanol groups, have a highly hydrophobic surface modification are obtained. With the use of these silicas, for example for rheology control of polar coatings or adhesives and sealants, this can lead to an extended incorporation time into the formulations in spite of unreacted silanol groups. Furthermore, the process is uneconomical owing to the high temperatures required.
The modification of finely divided silicas with sparingly volatile siloxanes is furthermore known (EP 686676). Here, highly hydrophobic silicas having a very small proportion of unreacted silanol groups are obtained. In polar coating materials, adhesives or sealants, the highly nonpolar surface siloxane layer can lead to more difficult wetting of the particles and, associated therewith, to an extended incorporation time of silicas into the liquid medium.
EP 924269 discloses the surface modification of silicas with dimethyldisilyloxy groups and monomethyltrisilyloxy groups. Here, cyclic siloxanes are reacted with silica at high temperatures (550-600° C.). Silicas having a high carbon content, i.e. better hydrophobization of the silica, result. The particles described there have a higher methanol number, i.e. a low polarity, compared with particles which were reacted at low temperature. Reaction at high temperatures is uneconomical owing to the energy costs. A disadvantage of these particles is that, although they have a lower proportion of surface silanol groups, they simultaneously also have a higher methanol number, i.e. a higher hydrophobicity.
The cosilylation of silica with dimethyldichlorosilane and methyltrichlorosilane is also known. What is problematic here is the substantially different reactivity of the silanes, which can lead to an inhomogeneous distribution of D and T groups, and the formation of large amounts of HCl, which has to be removed from the product by purification.